Explanation
Test techniques are methods or procedures that can be used to design, execute, or evaluate test cases. Test techniques can be classified into two categories: specification-based and structure-based. Specification-based test techniques, also known as black-box test techniques, are based on the requirements, specifications, or expectations of the system under test. They do not require any knowledge of the internal structure or implementation of the system. Some examples of specification-based test techniques are use case testing, state transition testing, decision table testing, etc. Structure-based test techniques, also known as white-box test techniques, are based on the code, architecture, or design of the system under test. They require some knowledge of the internal structure or implementation of the system. Some examples of structure-based test techniques are control flow testing, data flow testing, branch testing, statement testing, etc. You can find more information about test techniques in A Study Guide to the ISTQB Foundation Level 2018 Syllabus, Chapter
41.

Explanation
The general principles of testing state that testing can show the presence of defects, but not their absence.
Running the same test set more often will not increase the likelihood of finding new defects, unless the system or its environment changes. Therefore, statement A contradicts the general principles of testing. Statement B is true, as testing is context-dependent and should be tailored to the specific situation of a project. Statement C is also true, as early testing can help prevent defects and reduce rework. Statement D is true, as it reflects the Pareto principle or the 80/20 rule, which states that most defects are found in a small subset of a system's modules.
References: ISTQB Certified Tester Foundation Level Syllabus 2018, Section 1.2

Exploratory testing would be a good test technique to use when under severe time pressure, because it is a type of experience-based technique that allows testers to design and execute tests based on their intuition, knowledge, and skills, without following a predefined test plan or test cases . Exploratory testing can be useful when testing time is very limited, as it can help to find important defects quickly and adapt to changing requirements or situations . The other options are not good test techniques to use when under severe time pressure. Option B is not a good option, because structure-based testing is a type of technique that requires analysing the code or design of the software under test to measure aspects such as code coverage, complexity, or coupling . Structure-based testing can be time-consuming and may not be feasible or effective when testing time is very limited . Option C is not a good option, because specification-based testing is a type of technique that requires analysing the requirements or specifications of the software under test to design test cases that verify the expected functionality or behaviour . Specification-based testing can also be time-consuming and may not be suitable or efficient when testing time is very limited . Option D is not a good option, because use case testing is a type of technique that requires analysing the use cases or scenarios of the software under test to design test cases that verify the end-to-end user interactions and outcomes . Use case testing can also be time-consuming and may not be relevant or effective when testing time is very limited .